Electrochemical cells used to generate fluorine gas generally include an anode, a cathode, an electrolyte, an electrolyte-resistant container, and a gas separator. Anodes are typically fabricated from amorphous, nongraphitic carbon. Cathodes are typically fabricated from mild steel, nickel, or Monel.TM. alloy. Electrolyte is generally KF.multidot.2HF containing approximately 39 to 42% hydrogen fluoride. Gas separators segregate the generated hydrogen (formed at the cathode) from the generated fluorine (formed at the anode) thereby avoiding spontaneous, and often violent, hydrogen fluoride reformation.
Electrochemical cells of this general type are described in Rudge, The Manufacture and Use of Fluorine and Its Compounds, 1845, 82-83 (Oxford University Press, 1962).
The upper portion of a carbon anode is typically connected through a metal connection to a current source. This metal/carbon junction can be corroded during cell operation and the extent and speed of corrosion can depend on the location of the metal/carbon junction. For example, in some cells, the metal/carbon junction is within the cell housing but not immersed in the electrolyte. Other cells are arranged such that the metal/carbon junction is within the cell housing and also immersed in the electrolyte. In still other configurations, the metal/carbon junction is removed completely from the cell housing and is situated above the cell cover. See, e.g. U.S. Pat. No. 3,773,644.
Within the teachings of the conventional art, the limit of the current density at which anodes can be operated satisfactorily is a principal constraint in optimizing cell operations. Conventional fluorine cells are typically operated at anodic current densities of 80 to 150 ma/cm.sup.2. One difficulty associated with attempting to run conventional fluorine cells at higher current densities is that carbon is a relatively poor conductor, particularly when compared to many metals. This-leads to resistance heating when substantial current is passed through the carbon. If this resistance heating produces more heat than can be dissipated, the carbon temperature will be elevated and the carbon will react with elemental fluorine. This reaction is significant whenever the temperature exceeds about 150.degree. C. This reaction will eventually result in the destruction of the carbon portion by burning or by converting it to a doughy state often noted in the art. Resistance heating is also a concern at the carbon/metal interface where it can lead to higher temperatures and enhanced corrosion.
Resistance heating in carbon anodes can be reduced or eliminated by including a metal conductor that extends into the anode (See for example, U.S. Pat. Nos. 3,655,535, 3,676,324, and 4,511,440 and GB Patent No. 2 135 335 A). Copper, for example, has a conductivity of 4000 times that of carbon, and a copper insert of sufficient cross section extending a substantial distance into the anode can carry the entire anode current with no significant generation of heat.
Resistance heating also accelerates the corrosion at the metal-carbon junction. The attack on the carbon and the corrosion at the junction increase the resistance in the anode and junction. Such an increase in resistance increases the resistance heating in the anode and junction. The result is a monotonic increase in resistance heating and temperature and attack on the anode and carbon-metal interface.
Many metals, including copper and nickel, will corrode (through the well-known mechanism of bimetallic corrosion (an electrochemical phenomenon) when they are in contact with another metal, or carbon, and an electrolyte, such as KF-2HF. When a carbon anode with an interior metal conductor is used, molten KF-2HF will eventually penetrate through the pores of the carbon to contact the interior metal conductor and cause the metal to corrode through bimetallic corrosion. This electrolyte penetration will occur, at immersions greater than about ten cm, through the pores present in ordinary dense carbon or through the pores of carbon especially made to be porous. Such corrosion at the carbon-metal interface will cause an increase in resistance (as stated above) at this interface. This increase in resistance at this interface will lead to increased resistance heating at the interface and to an increased corrosion rate. Furthermore, the corrosion products from the metal occupy more volume than did the original metal. This increased volume leads to pressure against the carbon anode and eventually causes the carbon to break.